Use Ansible network roles

Roles are sets of Ansible defaults, files, tasks, templates, variables, and other Ansible components that work together. As you saw on Run Your First Command and Playbook, moving from a command to a playbook makes it easy to run multiple tasks and repeat the same tasks in the same order. Moving from a playbook to a role makes it even easier to reuse and share your ordered tasks. You can look at Ansible Galaxy, which lets you share your roles and use others' roles, either directly or as inspiration.

• Understanding roles
  • A sample DNS playbook
  • Convert the playbook into a role
  • Variable precedence
    • Lowest precedence
    • Highest precedence
  • Update an installed role

Understanding roles

So what exactly is a role, and why should you care? Ansible roles are basically playbooks broken up into a known file structure. Moving to roles from a playbook makes sharing, reading, and updating your Ansible workflow easier. Users can write their own roles. So for example, you don't have to write your own DNS playbook. Instead, you specify a DNS server and a role to configure it for you.

To simplify your workflow even further, the Ansible Network team has written a series of roles for common network use cases. Using these roles means you don't have to reinvent the wheel. Instead of writing and maintaining your own create_vlan playbooks or roles, you can concentrate on designing, codifying and maintaining the parser templates that describe your network topologies and inventory, and let Ansible's network roles do the work. See the network-related roles on Ansible Galaxy.

A sample DNS playbook

To demonstrate the concept of what a role is, the example playbook.yml below is a single YAML file containing a two-task playbook. This Ansible Playbook configures the hostname on a Cisco IOS XE device, then it configures the DNS (domain name system) servers.

---
- name: configure cisco routers
  hosts: routers
  connection: ansible.netcommon.network_cli
  gather_facts: no
  vars:
    dns: "8.8.8.8 8.8.4.4"

  tasks:
   - name: configure hostname
     cisco.ios.ios_config:
       lines: hostname {{ inventory_hostname }}   - name: configure DNS
     cisco.ios.ios_config:
       lines: ip name-server {{dns}}

If you run this playbook using the ansible-playbook command, you'll see the output below. This example used -l option to limit the playbook to only executing on the rtr1 node.

[[email protected] ~]$ ansible-playbook playbook.yml -l rtr1

PLAY [configure cisco routers] *************************************************

TASK [configure hostname] ******************************************************
changed: [rtr1]

TASK [configure DNS] ***********************************************************
changed: [rtr1]

PLAY RECAP *********************************************************************
rtr1                       : ok=2    changed=2    unreachable=0    failed=0

This playbook configured the hostname and DNS servers. You can verify that configuration on the Cisco IOS XE rtr1 router:

rtr1#sh run | i name
hostname rtr1
ip name-server 8.8.8.8 8.8.4.4

Convert the playbook into a role

The next step is to convert this playbook into a reusable role. You can create the directory structure manually, or you can use ansible-galaxy init to create the standard framework for a role.

[[email protected] ~]$ ansible-galaxy init system-demo
[[email protected] ~]$ cd system-demo/
[[email protected] system-demo]$ tree
.
    defaults
    main.yml
    files
    handlers
    main.yml
    meta
    main.yml
    README.md
    tasks
    main.yml
    templates
    tests
      inventory
    test.yml
       vars
         main.yml

This first demonstration uses only the tasks and vars directories. The directory structure would look as follows:

[[email protected] system-demo]$ tree
.
    tasks
    main.yml
       vars
           main.yml

Next, move the content of the vars and tasks sections from the original Ansible Playbook into the role. First, move the two tasks into the tasks/main.yml file:

[[email protected] system-demo]$ cat tasks/main.yml
---
- name: configure hostname
  cisco.ios.ios_config:
    lines: hostname {{ inventory_hostname }}- name: configure DNS
  cisco.ios.ios_config:
    lines: ip name-server {{dns}}

Next, move the variables into the vars/main.yml file:

[[email protected] system-demo]$ cat vars/main.yml
---
dns: "8.8.8.8 8.8.4.4"

Finally, modify the original Ansible Playbook to remove the tasks and vars sections and add the keyword roles with the name of the role, in this case system-demo. You'll have this playbook:

---
- name: configure cisco routers
  hosts: routers
  connection: ansible.netcommon.network_cli
  gather_facts: no

  roles:
    - system-demo

To summarize, this demonstration now has a total of three directories and three YAML files. There is the system-demo folder, which represents the role. This system-demo contains two folders, tasks and vars. There is a main.yml is each respective folder. The vars/main.yml contains the variables from playbook.yml. The tasks/main.yml contains the tasks from playbook.yml. The playbook.yml file has been modified to call the role rather than specifying vars and tasks directly. Here is a tree of the current working directory:

[[email protected] ~]$ tree
.
    playbook.yml
       system-demo
        tasks
        main.yml
           vars
               main.yml

Running the playbook results in identical behavior with slightly different output:

[[email protected] ~]$ ansible-playbook playbook.yml -l rtr1

PLAY [configure cisco routers] *************************************************

TASK [system-demo : configure hostname] ****************************************
ok: [rtr1]

TASK [system-demo : configure DNS] *********************************************
ok: [rtr1]

PLAY RECAP *********************************************************************
rtr1             : ok=2    changed=0    unreachable=0    failed=0

As seen above each task is now prepended with the role name, in this case system-demo. When running a playbook that contains several roles, this will help pinpoint where a task is being called from. This playbook returned ok instead of changed because it has identical behavior for the single file playbook we started from.

As before, the playbook will generate the following configuration on a Cisco IOS-XE router:

rtr1#sh run | i name
hostname rtr1
ip name-server 8.8.8.8 8.8.4.4

This is why Ansible roles can be simply thought of as deconstructed playbooks. They are simple, effective and reusable. Now another user can simply include the system-demo role instead of having to create a custom 'hard coded' playbook.

Variable precedence

What if you want to change the DNS servers? You aren't expected to change the vars/main.yml within the role structure. Ansible has many places where you can specify variables for a given play. See Using Variables for details on variables and precedence. There are actually 21 places to put variables. While this list can seem overwhelming at first glance, the vast majority of use cases only involve knowing the spot for variables of least precedence and how to pass variables with most precedence. See Variable precedence: Where should I put a variable? for more guidance on where you should put variables.

Lowest precedence

The lowest precedence is the defaults directory within a role. This means all the other 20 locations you could potentially specify the variable will all take higher precedence than defaults, no matter what. To immediately give the vars from the system-demo role the least precedence, rename the vars directory to defaults.

[[email protected] system-demo]$ mv vars defaults
[[email protected] system-demo]$ tree
.
    defaults
    main.yml
    tasks
    main.yml

Add a new vars section to the playbook to override the default behavior (where the variable dns is set to 8.8.8.8 and 8.8.4.4). For this demonstration, set dns to 1.1.1.1, so playbook.yml becomes:

---
- name: configure cisco routers
  hosts: routers
  connection: ansible.netcommon.network_cli
  gather_facts: no
  vars:
    dns: 1.1.1.1
  roles:
    - system-demo

Run this updated playbook on rtr2:

[[email protected] ~]$ ansible-playbook playbook.yml -l rtr2

The configuration on the rtr2 Cisco router will look as follows:

rtr2#sh run | i name-server
ip name-server 1.1.1.1

The variable configured in the playbook now has precedence over the defaults directory. In fact, any other spot you configure variables would win over the values in the defaults directory.

Highest precedence

Specifying variables in the defaults directory within a role will always take the lowest precedence, while specifying vars as extra vars with the -e or --extra-vars= will always take the highest precedence, no matter what. Re-running the playbook with the -e option overrides both the defaults directory (8.8.4.4 and 8.8.8.8) as well as the newly created vars within the playbook that contains the 1.1.1.1 dns server.

[[email protected] ~]$ ansible-playbook playbook.yml -e "dns=192.168.1.1" -l rtr3

The result on the Cisco IOS XE router will only contain the highest precedence setting of 192.168.1.1:

rtr3#sh run | i name-server
ip name-server 192.168.1.1

How is this useful? Why should you care? Extra vars are commonly used by network operators to override defaults. A powerful example of this is with Red Hat Ansible Tower and the Survey feature. It is possible through the web UI to prompt a network operator to fill out parameters with a Web form. This can be really simple for non-technical playbook writers to execute a playbook using their Web browser. See Ansible Tower Job Template Surveys for more details.

Update an installed role

The Ansible Galaxy page for a role lists all available versions. To update a locally installed role to a new or different version, use the ansible-galaxy install command with the version and --force option. You may also need to manually update any dependent roles to support this version. See the role Read Me tab in Galaxy for dependent role minimum version requirements.

[[email protected]]$ ansible-galaxy install mynamespace.my_role,v2.7.1 --force

See also

Ansible Galaxy documentation

Ansible Galaxy user guide

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